a) Field of the Invention
The present invention relates to a zoom lens system which comprises a front lens unit having a negative refractive power and a rear lens unit having a positive refractive power, and more specifically to a zoom lens which consists of a small number of lens elements.
b) Description of the Prior Art
Zoom lens systems of the type described above have conventionally been widely used as the so-called standard zoom lens system which cover standard field angles among exchange lens systems for use with single-lens reflex cameras. A zoom lens system of this type is configured so as to have the retrofocus type paraxial refractive power distribution by disposing a front lens unit having a negative refractive power apart from a rear lens unit having a positive refractive power, easily permits reserving a long back focal length which is required for disposing quick return mirrors in single-lens reflex cameras, has a compact size as a whole and features favorable optical performance.
As a conventional example of a zoom lens system which consists of two lens units i.e., a negative lens unit and a positive lens unit, and comprises a reduced number of lens elements, there is known a zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 59-64,811. This zoom lens system has a focal length of approximately 35 to 70 mm. The front lens unit consists of two lens components of two lens elements, whereas the rear lens unit consists of four lens components of four lens elements; the front lens unit using an aspherical surfaces for correcting aberrations in the zoom lens system.
Further, a zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-46,308 is known as another example of a zoom lens system which consists of a reduced number of lens elements. This zoom lens system also has a focal length of approximately 35 to 70 mm. This zoom lens system uses a front lens unit composed of two lens components of two lens elements, a rear lens unit composed of two lens components of two lens elements or three lens components of three lens elements and four or more aspherical surfaces for correcting aberrations in the zoom lens system.
As a conventional example of a zoom lens system which is of the type described above and to be used with lens shutter cameras, there is known a zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-50,718. Since zoom lens systems which are to be used with lens shutter cameras need not have long back focal lengths, these lens systems can have shortened total lengths (lengths as measured from first surfaces of the lens systems to film surfaces). In case of this conventional example, a total length thereof is shortened by composing a rear lens unit thereof, in order from the object side, of a subunit having a positive refractive power and another subunit having a negative refractive power, and disposing these subunits apart from each other so as to obtain the so-called telephoto type refractive power distribution. This zoom lens system has a focal length of approximately 35 to 70 mm, and uses three lens components of three lens elements to compose the front lens unit and four lens components of six lens elements for composing the rear lens unit as well as aspherical surfaces disposed in these lens units for correcting aberrations in the zoom lens system.
The zoom lens system disclosed by the above-mentioned Japanese Patent Kokai Publication No. Sho 59-64,811 comprises as a whole lens elements in a number as small as six. However, due to the refractive power distribution selected for this zoom lens system, the front lens unit thereof has a large effective diameter which is undesirable for configuring the lens system compact. In addition, the two lens elements disposed in the front lens unit have large effective diameters, thereby undesirably enhancing costs of materials and manufacturing of the zoom lens system. The effective diameters are further prolonged in particular when the zoom lens system is configured so that it has a short focal length and a large field angle at a wide position thereof.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-46,308 consists of four or five lens elements as a whole, but has aberrations which are not corrected sufficiently favorably for practical use of the zoom lens system.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-50,718 has a shortened total length and a shortened effective diameter, but is undesirable from a viewpoint of manufacturing cost thereof since the lens system uses nine lens elements as a whole.